1. Field of the Invention
This invention relates to a distributed antenna system for carrying out a communication between an antenna and a mobile terminal by means of carrier aggregation using a low-frequency band and a high-frequency band.
2. Description of the Related Art
In a mobile communication system, there are a base station and a mobile terminal that use plural frequency bands such as 800 MHz band and 2 GHz band.
As shown in FIG. 4, in the base station using the plural frequency bands, a wireless system 21a for the 800 MHz band and a wireless system 21b for the 2 GHz band are connected to a single antenna 20. The wireless systems 21a, 21b are respectively equipped with RF modules 22a, 22b and BB modules 23a, 23b. In each of the RF modules 22a, 22b, a radio frequency (RF) signal handled at the antenna 20 and a base band (BB) signal handled at the BB modules 23a, 23b are mutually converted.
In the mobile communication system, for example, a communication is normally carried out by using the 800 MHz band, and when the 800 MHz band is crowded, the communication is carried out by that the used frequency is switched to the 2 GHz band.
The switch of frequency band is carried out by switching to the wireless system 21a or the wireless system 21b, namely to any one of the wireless system 21a for the 800 MHz band and the wireless system 21b for the 2 GHz band as the situation demands.
By the way, different frequency bands have different transmission loss, and a radio wave of the 2 GHz band of high-frequency band has a narrower communication range than a radio wave of the 800 MHz band of low-frequency band, so that in particular, it is difficult for the high-frequency band to reach cell edges of the communication range of low-frequency band.
Thus, a distributed antenna system has been developed, the antenna system having a configuration that plural antennas for the high-frequency band are disposed so as to cover a communication range of the low-frequency band, thereby a communication can be carried out anywhere within a predetermined communication range at both of the low-frequency band and high-frequency band.
The plural antennas for the high-frequency band are disposed in such a way that first a base station is respectively installed in each place, and the antennas for the high-frequency band are connected to BB modules of each base station via RF modules.
JP-A-2000-23238 and JP-A-H6-90198 disclose a communication system that includes a base station of a wide communication range and plural base stations of a narrow communication range covering the wide communication range, and selects any one of the base station of a wide communication range and the base station of a narrow communication range in accordance with the communication speed and moving speed of the mobile body.